The present invention relates to an electrophotographic copying machine and, more particularly, to an abnormal condition detection device for a corona discharger in an electrophotographic copying machine.
An electrophotographic copying machine produces on a photoconductive layer such as a master paper an electrostatic latent image corresponding to a pattern image on a document such as a manuscript or book to be copied. Toner particles are electrostatically adhered to the latent image, so that the latent image becomes visible to form a toner image. The toner image is transferred from the photoconductive layer to a copy paper via a transference corona discharger.
A conventional transference corona discharger comprises means for causing the photoconductive layer to be charged in a specific polarity by applying a high voltage to a thin tungsten wire to cause corona charging. To apply the high voltage to the tungsten wire, a high voltage transformer is needed for receiving an AC input voltage at a primary winding and outputting an AC high voltage being N times as great as the input voltage from a secondary winding, so that the AC output is directly or, after being rectified, applied to the wire.
Recent development of semiconductor technology enables the charging high voltage transformer comprise a DC-DC converter or a DC-AC converter having a semiconductor circuit. This assists in assuring that a load current or voltage is kept constant to keep a surface level of the photoconductive layer constant even when the secondary-winding load of the discharger is unstable or the input voltage to the charging high voltage transformer is changed.
Nevertheless, since the charging wire of the corona discharger is very thin, the wire may be easily broken or short-circuited. When a plurality of corona dischargers are operated and at least one of them is damaged owing to the broken or short-circuited wire, the thus damaged corona discharger is unbalanced with the remaining dischargers, so that the surface level of the photoconductive layer is abnormally high or low. As a result, the photoconductive layer is damaged thereby reducing its copying function and shortening its operative life.